1. Corn
Corn is the most important crop grown in the United States. Corn is sometimes called maize and has the scientific name Zea mays. Corn has a growing season of about four to six months. A mature corn plant consists of a stalk with an ear of corn. The ear of corn consists of about 800 kernels, or seeds, on a cylindrical cob. The kernels are eaten whole and are also processed into a wide variety of food and industrial products.
In more detail, the corn kernel consist of three main parts: (1) the pericarp; (2) the endosperm; and (3) the germ. The pericarp (also known as the seed coat or bran) is the outer covering of the kernel. It consists primarily of relatively coarse cellulosic fiber. The endosperm is the energy reserve for the plant. It contains starch, protein (also known as gluten), and small amounts of relatively fine cellulosic fiber. The distribution of protein and fiber in the endosperm is not uniform. The starch in the endosperm that is in close contact with the protein and fine fiber is commonly known as “hard” starch and this portion of the endosperm is yellow in color. The hard starch, protein, and fine fiber form what is commonly known as a matrix. The starch in the endosperm that is substantially free of the protein and fine fiber is commonly known as “soft” starch and is white in color. The germ (also known as the embryo) is a miniature plant with a root-like portion and several embryonic leaves. The germ also contains oil.
A corn kernel generally contains about 10 to 30 percent water at the time of harvest. In the United States, a water content of about 15 percent is considered standard. On a dry substance basis, a corn kernel contains about 80 percent starch, 12 percent protein, 6 percent oil, and 2 percent fiber. All percentages herein are based on weight unless otherwise noted.
Starch is stored in the kernel in the form of discrete crystalline bodies known as granules. Starch granules have a diameter of about 5 to 25 microns. Starch granules are insoluble in water. However, if the granules are heated in water at a temperature of at least about 125° F. (52° C.), subjected to shear, or subjected to other treatments, they can lose their crystalline structure and form a gel or paste. This process is commonly known as gelatinization or pasting. Once starch has gelatinized or pasted, it can never return to the crystalline granular structure.
On a molecular level, starch is a polymer of anhydroglucose units (C6H12O6). Starch is a member of the general class of carbohydrates known as polysaccharides. Polysaccharides contain multiple saccharide units, disaccharides contain two saccharide units, and monosaccharides contain a single saccharide unit. The length of a saccharide chain (the number of saccharide units in it) is sometimes described by stating its “degree of polymerization” (abbreviated to D.P.). Starch has a D.P. of 1000 or more. Maltose is a disaccharide (its D.P. is 2) that is composed of two glucose units. Glucose (also known as dextrose) is a monosaccharide (its D.P. is 1). Saccharides having a D.P. of about 5 or less are sometimes referred to as sugars. The solubility of saccharides in water generally increases as the D.P. decreases. The distribution of the varying D.P. sugars in a mixture is frequently expressed as a dextrose equivalent (DE) which is the total reducing power of all the sugars present relative to glucose as 100.
The saccharide units in starch are connected to each other in one of two ways. When connected together in alpha-1,4-linkages, the starch molecule is linear. When connected together in alpha-1,6-linkages, a branch occurs. The relative number of the two linkages varies depending on the variety of corn. Both types of linkages are sometimes referred to as glucosidic linkages. Cellulose is also a polysaccharide formed of glucose units. However, the glucose units in cellulose are connected together in beta-1,4-linkages which cannot be broken down in the human digestive system.
2. Dry Milling Process
A wide variety of processes have been used to separate the various components of corn. These separation processes are commonly known as corn refining. Commercial corn refining processes do not make a precise separation of the components. In other words, each component contains some of one or more of the other components. The cost to make a more precise separation is inevitably outweighed by the economic benefits. One of the earliest processes developed is commonly known as the dry milling process.
In a typical dry milling process, the corn kernels are first cleaned and then soaked in water to increase their moisture content. The softened corn kernels are then ground in coarse mills to break the kernel into three basic types of pieces—pericarp, germ, and endosperm. The pieces are then screened to separate the relatively small pericarp and germ from the relatively large endosperm.
The pericarp and the germ are then separated from each other. The germs are then dried and the oil is removed. The remaining germ is typically used for animal feed. Meanwhile, the endosperm is ground to make corn flakes. The corn flakes contain most of the starch and protein from the kernel.
3. Wet Milling Process
A different process was developed to isolate granular starch from corn. The process is commonly known as the wet milling process. After isolation, the granular starch is processed in one of many different ways. The starch can be dried and sold as unmodified starch. The starch can be modified and used for food or industrial purposes. The starch polymer can be partially shortened to produce corn syrup or shortened all the way to the individual glucose (dextrose) units. The partial shortening process is commonly known as liquefaction because the resulting fragments are water soluble. The process of shortening all the way to glucose is commonly known as saccharification. If shortened all the way to glucose, the glucose molecules can be isomerized to fructose. Fructose is considerably sweeter than glucose and is widely used in the food industry.
In a typical wet milling process, the corn kernels are first cleaned and then soaked for 24 to 48 hours in warm water containing sulfurous acid (H2SO3). The temperature of the water is generally less than 125° F. (52° C.) so the starch does not gelatinize. The sulfurous acid controls fermentation and also helps in the physical separation of the components that occurs later. This soaking step is commonly known as steeping. During steeping, water soluble proteins and other substances dissolve into the steepwater. These components are then recovered from the steepwater.
After steeping, the softened corn kernels are ground in coarse mills to break the kernel without damaging the germ. The kernels then flow to centrifugal separators which separate the less dense germs from the denser pericarp and endosperm. The germs are then dried and the oil is removed.
The pericarp and endosperm are then ground in fine mills. The finely ground stream flows to screens which separate the small particle size pericarp from the larger particle size endosperm. The endosperm stream then flows to centrifugal separators that separate the less dense protein from the denser starch. The finished starch is in granular form and is suitable for many different types of further processing.
The wet milling process is effective at isolating starch in the granular form. Many commercial processes are able to isolate over 90 percent of the starch in the kernel. In other words, less than 10 percent of the starch ultimately ends up in other components. However, the use of sulfurous acid causes many environmental problems. Furthermore, the process uses large quantities of water and produces large quantities of waste water that must be treated.
4. Enzymes
When the wet milling process was first developed, the corn starch was liquefied and saccharified by treatment with acids at relatively high temperatures and pressures if reduced D.P. products were desired. Within the past few decades, the corn refining industry has largely switched to the use of enzymes instead of acids. An enzyme is a protein formed by living cells that catalyzes a broad spectrum of biochemical reactions. Enzymes are very specific in that they catalyze one particular reaction and no others. The use of enzymes enables liquefaction and saccharification to be conducted at lower temperatures and pressures which, in turn, reduces various undesired reactions of the starch. These undesired reactions reduce the yield of desired products and also produce reaction products that contribute off-colors and flavors.
Enzymes that are commonly used in corn refining include: (1) glucosidic linkage cleaving enzymes that break down the glucosidic linkages in starch; (2) protease enzymes that break down the peptide linkages in protein; and (3) cellulase enzymes that breaks down the beta-1,4-linkages in cellulosic fiber. The most common glucosidic linkage cleaving enzymes are: (1) amylase enzymes that break down the linear alpha-1,4-linkages and the branched 1-6-linkages in the starch molecule; (2) pullanase enzymes that break down the branched 1,6-linkages only; and (3) maltase enzymes that break down maltose into two glucose molecules. The most common amylase enzymes are: (1) alpha-amylase that is added in the liquefaction step to split the starch molecule into soluble fragments; and (2) glucoamylase (also known as glucohydrolyase or amyloglucosidase) that is added in the saccharification step to break the fragments into glucose. Some of these enzymes are naturally present in the corn (e.g., maltase) while others are added by the refiner.
The activity of enzymes is dependent on temperature, pH, and other factors. For example, an enzyme that has its maximum activity at 65° C. has only about half that activity at 55° C. and at 75° C. To a large extent, enzyme manufacturers have tried to supply enzymes to the corn refining industry that will function at the process conditions. However, the conditions are rarely optimal for the enzymes.
5. Ethanol Processes
Fermentation is a process by which microorganisms such as yeast digest sugars and starches to produce ethanol and carbon dioxide. The basic reaction isC6H12O6→2C2H5OH+2CO2 Yeast reproduce aerobically (oxygen is required) but can conduct fermentation anaerobically (without oxygen). The fermented mixture (commonly known as the beer mash) is then distilled to recover the ethanol. Distillation is a process in which a liquid mixture is heated to vaporize the components having the highest vapor pressures (lowest boiling points). The vapors are then condensed to produce a liquid that is enriched in the more volatile compounds.
Various fractions of corn have been used for fermentation, including the entire kernel, just the endosperm, and just the starch. For example, a process based on the dry milling process in which all the components of the kernel are fed to fermentation is illustrated in FIG. 1. A process based on the wet milling process in which granular starch is isolated and then optionally fed to fermentation is illustrated in FIG. 2. Processes have also been disclosed in which the starch is liquefied and saccharified before fermentation.
For example, Keim, U.S. Pat. No. 4,361,651, Nov. 30, 1982, discloses a corn refining process in which the corn is steeped, milled, and degermed. The degermed corn is then liquefied and saccharified. The pericarp and the gluten are then removed and the glucose solution is fermented. The fermented mash is then distilled to recover ethanol. The Keim process is based upon the wet milling process and includes the steeping, milling, and degerming steps. Accordingly, the Keim process suffers from the disadvantages of the wet milling process, including environmental problems caused by the use of sulfurous acid and the production of large quantities of waste water.
As another example, Singh et al., U.S. Pat. No. 6,254,914, Jul. 3, 2001, discloses a corn refining process in which the corn is soaked in hot water (preferably distilled water for 12 hours), milled, and degermed. The pericarp is then removed and the resulting stream containing starch, protein (gluten), and fine fiber is then ground and saccharified. The mixture is then fermented. The fermented mash is then distilled to recover ethanol. The Singh et al. process is relatively slow and it produces large quantities of waste water.
Accordingly, there is a demand for a corn refining process that quickly and efficiently produces ethanol without the use of sulfurous acid and without the production of large quantities of waste water. There is also a demand for a corn refining process that is conducted at optimal conditions for the enzymes. There is further a demand for a corn refining process in which a variety of components can be isolated as economic conditions dictate without changing the basic steps.